Generally, a sand basin, a sedimentation basin, screening equipment, or the like has conventionally been adopted for removal of suspended matters and/or sediment contained in liquid.
In a system using the sand basin where the sand tends to involve a large amount of organic substances, the sand used may be treated by means of sand-lifting or sand-washing to separate such organic substances therefrom. However, the these means for the separation; is unacceptable with the use of complicated and costly mechanical means. The sedimentation basin to be used in a waste water treatment system will require a large area thereof for the removal of, particularly, substances of a low specific gravity and additionally a complex and costly mechanical means for the removal. With a system using the screening equipment, screens having various openings must be provided for removing suspended matters of various sizes, e.g. microscopic matters, coarse matters, and these screens must be exchanged as the occasion demands. Such frequent and complex work for the exchange is, however, impractical.
In view of the foregoing, the present inventor has proposed an arrangement where liquid subject to treatment is caused to create a circular flow thereof to settle sedimentary substances for removal by making use of their hydraulic characteristics (disclosed, for example, in Japanese Patent Laid-open No. 171,718/1987). The present inventor has proposed another arrangement including a baffle plate within a processing vessel to restrain the surface flow of liquid circular flow and an overflow spout located on a side wall of the vessel at the upstream side of the baffle plate, to restrain the migration of suspended matters and/or organic substances carried by the surface flow of the liquid and to withdraw them from the overflow spout (disclosed, for example, in Japanese Patent Laid-open Patent No. 144,895/1988). With such arrangements utilizing the hydraulic characteristics of the suspended matters, organic substances, and sediment, the suspended matters and organic substances are floated and the sediment is settled so that they may be withdrawn respectively in an efficient manner. However, liquids such as clean water originated from river surface water as a water source, rain water appeared in city areas when it rains, sewage or drainage caused by human living, and industrial effluent, sometimes include a lot of crude refuse and suspended matters or sediment and solid materials, or both, when high rainfall flashes (these crude refuse and suspended matters will be hereinafter simply designated as "suspended matter", and these sediment and solid matters will be hereinafter simply as "sediment", as the case may be). It has been therefore desired to develop equipment and a treating method for removing such a lot of suspended matters or sediment, or both from liquid in more efficient manner, temporarily storing these waste which may, as a consequence, be released if necessary, and providing facilities for cleaning of a storing zone in spite of these conditions.